The use of rising numbers of channels in the transmission of an optical wavelength-division multiplex (WDM) signal along lengthy fiber links leads, in conjunction with the same total output power of the optical amplifiers, to a reduction in the output power in the fiber per transmission channel and, thus, to a worsening of the optical signal-to-noise ratios OSNR at the receiver, which are decisive for the achievable bit error rate BER. A similar problem arises with increasing the data rate. In order to achieve an identical system performance after raising the data rate, a higher OSNR must be achieved at the receiver than with a lower data rate. Achieving a sufficient OSNR at the receiver is, therefore, a critical point in the design of future system generations.
Optical amplifier modules are required downstream of respective transmission sections for the purpose of transmitting an optical WDM signal along lengthy fiber links. An effective method for additionally amplifying a signal is based on stimulated Raman scattering, in the case of which a pumping signal is fed into the transmission fiber. The pumping signal can be generated in this case via a number of pumping sources; namely, laser diodes.
The prior art known in this context is explained in more detail below with the aid of FIGS. 1 to 3.
As shown in FIG. 1, the use of a number of pumping wavelengths leads to a broad flat gain spectrum in the C-band and L-band.
The wavelength set of the pumping sources is set such that all the channels of the WDM signal are amplified as identically as possible, taking account of the Raman gain spectrum (see “Fiber Optic Communication Systems”, G. P. Agrawal, 2nd edition, page 381, FIG. 8.11). A channel with a frequency shift of 13.2 THz relative to a pumping frequency is amplified to the maximum. If there is a smaller or larger frequency difference between a channel and a pumping signal, the channel is amplified less. By using a relatively large number of different pumping wavelengths, all the channels of the WDM transmission signals are amplified more homogeneously.
Such a Raman amplifier is described, for example, in a prior German patent application with the file reference P 10048460.3.
Mach-Zehnder interferometers, which permit operation for launched powers of up to 2 W, for example, are often used for multiplexing the various pumping wavelengths. This requires a pumping wavelength array with pumping wavelengths that are equidistant from one another. A detailed description is given in the publication “Namiki et al., Proc. OAA 2000, Quebec, OMB 2, 7-9”. It is also possible to use interference filters when multiplexing a small number of pumping wavelengths. In this case, non-equidistant spacings of the pumping wavelengths also can be achieved. The power launched in such multiplexers is, however, lower than in the case of Mach-Zehnder interferometers. In the publication “Kidorf et al., IEEE Phot. Technol. Lett., 11 (1999), 530-532”, there is a description of non-equidistant distribution of the pumping wavelengths in the case of which a greater concentration of the smaller pumping wavelengths is provided by comparison with larger pumping wavelengths. A corresponding power transfer from small to larger pumping wavelengths is compensated as a result by Raman interaction along the fiber.
Four-wave mixing FWM occurs between the pumping wavelengths in specific fiber types, primarily in the case of low dispersion in the region of the pumping wavelengths, when use is made of equidistant pumping wavelengths, such as (λ1, . . . , λ8) in FIG. 2, for signal transmission in the C-band and L-band; that is, pumping wavelengths between approximately 1 420 nm and 1 510 nm, signal wavelengths between approximately 1 525 nm and 1 610 nm. Consequently, new frequency components, or what are termed mixing products MPi (i>0), are generated in the case of sums or differences of pumping frequencies that are superimposed in or outside the spectrum of the pumping source (see FIG. 2). The mixing products can, therefore, be superimposed directly on the WDM signal spectrum if the higher pumping wavelengths are near the smaller signal wavelengths. The signal quality, such as the signal-to-noise ratios OSNR of specific channels of the WDM signal, is thereby worsened. The levels of the WDM signal in the wavelength region are illustrated in FIG. 3. As illustrated in FIG. 3, four-wave mixing leads to signal-to-noise ratio OSNR differences of 8 dB owing to the superimposition of mixing products in the signal spectrum. The described effect occurs particularly strongly with a broadband Raman amplification in the C-band and L-band, since the pumping wavelengths must reach close to the C-band for an amplification in the L-band. The strongest mixing products are, therefore, situated in the C-band and are superimposed on the signal spectrum.
A detailed description of four-wave mixing is given in “Agrawal, Nonlinear Fiber Optics, 1995, page 404”. In this context, there is a description here of the differences between a degenerated and a non-degenerated four-wave mixing FWM. The non-degenerated four-wave mixing is based on the interaction of photons of three different wavelengths that produce a photon at a fourth wavelength, while in the case of the degenerated four-wave mixing, a wavelength features in a quasi-doubled fashion in the mixing process. The non-degenerated four-wave mixing, therefore, requires three different wavelengths, while the degenerated four-wave mixing can be performed given two wavelengths. The strongest mixing product from four-wave mixing in the case of a broadband pumping source with a number of pumping wavelengths (λ1, . . . , λm) is thus located at a spacing of λm-λm−1 after the highest pumping wavelength λm in the case of degenerated four-wave mixing.
It is, therefore, an object of the present invention to eliminate, or at least strongly minimize, the influence of the mixing products in the signal spectrum that are caused by four-wave mixing FWM in a broadband pumping source.